Even thought this exercise has the word negative in it, doesn't mean it isn't good. Men's Health shows you exactly how to do the "negative crunch" exercise, where you need to lean backwards into a 45º angle and work your abs.
Sit with your knees bent and your feet flat on the floor shoulder-width apart. Tuck your feet under weights to maintain balance and extend your arms in front of you. Begin with your upper body at a slightly less-than-90º angle to the floor.
Lower your upper body to the floor curling your torso forward, rounding your lower back and keeping your abs contracted. When your upper body reaches a 45º angle to the floor, return to the starting position.
Try not to apply too much pressure to the dumb-bells with your toes as this can put stress on your hip flexors instead of the target muscle – the abdominals. Breathe in as you lie back and breathe out as you return to the starting position, maintaining technique throughout.